Lipids are hydrophobic or amphipathic small molecules that originate entirely or in part by carbanion-based condensations of thioesters like fatty acids or polyketides and/or by carbocation-based condensations of isoprene units like prenols or sterols. This group comprises several substances of high economical importance. The group of the triacylglycerol-lipids comprises, for example, oils, fats or waxes, which are used in a vast variety of contexts, e.g. as ingredients in food or for cooking, to make soaps, skin products, perfumes and other personal care and cosmetic products, for making paints and other wood treatment products, as biodegradable insulators in the electrical industry, to produce bio-degradable hydraulic fluids, as lubricants, or even as the basis of biodiesel, which can be used to substitute conventional diesel. The isoprenoid-lipid squalene is, for instance, used as an adjuvant in vaccines or other pharmaceuticals, nutrients, cosmetics as well as over-the-counter drugs. Squalene can also be used as a building block for the synthesis of terpenes. Furthermore squalene can industrially be used as biodegradable lubricant. Other economically important lipids are sterols like ergosterol, zymosterol, episterol, 7-dehydrocholesterol or lanosterol, which are utilized as pivotal starting material for the production of compounds like saponin, steroid hormones, vitamins and pharmaceutical substances.
Lipids, in particular neutral lipids, are normally stored in the cell in specific intracellular organelles known as lipid particles. These particles are characterized by a simple structure consisting of a highly hydrophobic monolayer with only a small amount of proteins embedded. The lipids are stored in the lipid particles until hydrolysis directs return of their components to metabolic and/or catabolic pathways. This process of lipid depot formation is widely used in nature, and all types of eukaryotic cells contain intracellular lipid particles, which can also be called lipid bodies, lipid droplets, oil bodies or oleosomes. In the yeast Saccharomyces cerevisiae lipid droplets accumulate up to 70% of the total lipid content of the cell.
Since most of the mentioned lipids are derivable from natural sources like plants, animals or microorganisms, attempts were made to increase the lipid amount in the living cells, i.e. to accumulate more lipids in the lipid particles. For this purpose it is known to modify the metabolic pathways of organisms. The document EP-0 486 290 A describes the over-expression of genes of the ergosterol metabolism in yeast resulting in an increase of the amount of ergosterol in the cells. The document WO03/064652 A discloses a method for the production of zymosterol based on the increase of lanosterol-C14-demethylase and HMG-CoA-reductase activity. In the document WO2004/083407 A transgenic organisms are described, which have a reduced Δ22-desaturase activity and an increased activity of HMG-CoA reductase, lanosterol C14-demethylase, squalene epoxidase and squalene synthetase. These organisms can be used for the production of the sterol lipid ergosta-5,7-dienol. Thus, by biologically modifying lipid generating organisms the yield of the produced lipids can be significantly increased.
However, the obtained lipid compounds are mostly unpure and intermingled and need to be separated and/or purified, in particular if the lipid is to be used as a starting material for further chemical syntheses or modifications. In addition, any purification procedure, especially if it is carried out on an industrial scale, is expensive, laborious and tends to pollute the environment.